Alkali metal energy conversion device

ABSTRACT

Alkali metal energy conversion device comprising sealing components made of a metal or metal alloy substrate. A solid phase bonded coating of aluminum/silicon alloy is provided on an internal surface thereof. The aluminum/silicon alloy coating has the composition by weight of 0-0.5% Mg, 0-0.4% Cu, 0-1.0% Fe, 0.5-4.0% Si, up to 0.5% other trace elements, some oxygen present as an oxide and remainder Al.

This invention relates to alkali metal energy conversion devices, suchas for example alkali metal cells and particularly sodium sulphur cells.Such cells typically employ a solid electrolyte element separatingcathodic and anodic reactants which are liquid at the cell operatingtemperature.

A known construction of device comprises an external casing, a solidelectrolyte element dividing the interior of the casing into twoelectrode regions, an electrically insulating element joined to theelectrolyte element, and at least one metal member sealed to theinsulating element. This structure of sealing components typically formspart of the sealing arrangement for the device, sealing off the twoelectrode regions both from each other and from the ambient environment.For example, the external casing of the device may be of metal, so thatany sealing of an electrode region requires a seal to be made betweenthe metal of the casing and the electrolyte element. However the metalof the casing must be electrically insulated from the electrolyteelement and the insulation is provided by the intervening electricallyinsulating element.

It will be appreciated that there are a number of options for the designof the external casing, in particular the choice of material to be used.

For example, reference may be made to U.S. Pat. No. 4,546,056 whichshows an alkali metal energy conversion device having a particulararrangement of external casing, in this case comprising a cup shapedinner housing made of aluminium separated from a steel outer housingelement by a layer of electrical insulation. The aluminium inner housingelement serves as an electrical conductor whilst the steel outer housingelement effectively provides support for the inner housing element.Aluminium has a number of properties which make it attractive for use insuch alkali metal devices, for example low density, high electricalconductivity and resistance to corrosion from, for example, species suchas polysulphides. Corrosion resistance against such species is due tothe formation of a layer of aluminium sulphide, which is, however,electrically insulating. In some applications, it is know to arc, plasmaor flame spray the surface of an aluminium can with a material whichforms an electrically conductive layer in such an environment. Nichromeis often used for this.

It will be appreciated that an aluminium can cannot be used on its ownfor the external casing since the substantial overpressures generatedwould necessitate a can of substantial thickness and therefore cost.U.S. Pat. No. 4,546,056 addresses this by providing a thin aluminium cansupported by an outer steel housing.

It is well known to hot dip aluminise steel to provide a compositematerial which is not only relatively economic to produce but alsohighly resistant to corrosion and sufficiently strong. Hitherto, suchmaterials have not been employed in alkali metal energy conversiondevices because they posess a number of severe drawbacks. The drawbacksassociated with hot dipped aluminised material stem from there being aninter-metallic structure which is inevitably formed between the steeland the aluminium during coating. This inter-metalic structure isessentially columnar and comparatively brittle. Consequently, it is notpossible to diffusion bond to a hot dip aluminised steel material since,at the temperatures involved, the inter-metallic alloy layer is weak andthe material is unable to sustain the stresses placed on it. As aresult, the material simply breaks away if diffusion bonding isattempted. A further problem is that hod dipped aluminised materials canbecome porous and thus allow corrosion.

Typically, the aluminium used in the hot dip process has a siliconcontent of approximately 10-14%. This has the effect of reducing theprocessing temperature required to coat the steel substrate by up to 60°C. The silicon is also used to aid in the formation of a more uniformintermetallic layer between the aluminium and steel. However, the use ofsuch high levels of silicon, although acting as a processing aid, hasbeen found to degrade the ability of the material to withstandtemperatures above 550° C.

In view of the inherent problems with hot dipped aluminised materials, avariety of other aluminium coating processes have been tested such asplasma arc or flame sprayed coatings. All have been found to havedeficiencies, particularly porosity, which render them unsuitable. As aconsequence, attention has turned to chromised steels. Although thesesatisfy many of the requirements, the necessity of using high carbonsteel produces a material of only limited ductility so that the externalcasing of an energy conversion device can only be drawn to a limitedextent. Alloys such as Inconel 600 and Fecralloy have also been used forthe external casings of metal energy conversion devices, but are costly.

In accordance with the present invention an alkali metal energyconversion device comprises sealing components made of a metal or metalalloy substrate having a solid phase bonded coating of aluminium/siliconalloy on an internal surface thereof, said aluminium/silicon alloycoating having the composition by weight of

0-0.5% Mg,

0-0.4% Cu,

0-1.0% Fe,

0.5-4.0% Si,

up to 0.5% other trace elements, some oxygen present as an oxide andremainder Al.

The composition of the aluminium/silicon alloy coating has been found tobe critical if both good corrosion resistance and high temperaturethermal stability are to be achieved. To this end it has been found thatan aluminium silicon alloy having the above composition satisfies bothcriteria.

The aluminium/silicon alloy coating may be bonded to the metal substrateat room temperature. Techniques for this include roll bonding andfriction surfacing. All are described as "solid phase" bondingtechniques in as much as the coating is not applied in molten condition.Such techniques avoid the formation of a brittle intermetallic layer. Apreferred bonding process is roll bonding. It should be noted howeverthat as a consequence of the roll bonding process a thin layer of almostpure Si may be deposited on the outer surfaces of the aluminium/siliconalloy. This is not included in the chemical analysis of the coating. Thepure Si layer comes from a releasing agent used in the cold rollingprocess.

The bond between the alloy coating and the substrate is considerablystronger than the bond formed by hot dip aluminising. In addition, ithas been found that the roll bonded coating has low porosity.

The use of an aluminium silicon alloy as the coating material offers thebenefit, over the more usual commercially pure aluminium roll bondedcoating, of giving a much more corrosion resistant layer. The mechanismfor this improved resistance is not fully understood but it has beenpostulated that it is due to the Si producing or aiding in the formationof a more stable or tenacious sulphide coating on the surface of thealuminium.

In another aspect the invention provides an alkali metal energyconversion device having an external casing, a solid electrolyte elementin the casing to divide the interior into electrode regions, anelectrically insulating element joined to the electrolyte element andmeans secured to the insulating element and the external casing to sealoff one of the electrode regions, wherein at least part of the externalcasing is made of a metal or metal alloy substrate having a solid phasebonded coating of aluminium/silicon alloy on an internal surfacethereof, said aluminium/silicon alloy coating having the composition byweight of:

0-0.5% Mg,

0-0.4% Cu,

0-1.0% Fe,

0.5-4.0% Si,

up to 0.5% other trace elements, some oxygen present as an oxide andremainder Al.

The means secured to the insulating element may also comprise componentsmade of said substrate having said coating, as may any furthercomponents exposed to corrosive species.

The substrate is preferably iron or nickel based and conveniently steel.The preferred bonding process is again roll bonding.

The invention is therefore based on the finding that the prior artteaching against the use of aluminium/steel composites for suchcomponents on the grounds of serious deficiencies was incorrect inrespect of room temperature bonded composites having the above definedformulation.

An important further development of this invention is to make the abovereferred coating of aluminium/silicon alloy relatively thick, that ismore that 50 microns. Such thicker coatings are especially useful sinceit has been found that the life of alkali metal energy conversiondevices, particularly sodium/sulphur cells, can be extended by formingthe coating of adequate thickness. Preferably the coating thickness isequal to or greater than 60 microns and more preferably equal to orgreater than 100 microns.

An Example of the invention will be described with reference to theaccompanying drawing in which:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagrammatic longitudinal cross section through a knownarrangement of a sodium sulphur cell.

Referring to FIG. 1 of the drawings, a sodium sulphur cell of thecentral sodium type is illustrated comprising a cylindrical beta aluminaelectrolyte tubular element 14 which is integrally closed at one end asshown and has its other end closed by an alpha alumina end plate 15. Theend plate 15 is sealed, by glazing to one end of the electrolyteelelment 14 and provides electrical insulation as well as a mechanicalseal. Within the sealed assembly there may be either an iron foilelement (not shown) or a mesh element (not shown) closely adjacent theinner cylindrical surface of the electrolyte tube 14 to leave acapillary region adjacent that surface constituting a wick. The interiorof the assembly is filled with sodium 20 which is liquid at theoperating temperature of the cell; the capillary maintains a layer ofliquid sodium over the inner surface of the electrolyte tube 14. Acurrent collector rod 21 extends into this sodium, passing through anaperture 22 in the alpha aluminia element 15.

Around the outside of the cylindrical portion of the electrolyte element14 is a cathode structure of annular form constituted by threethird-cylindrical elements 23 and a further cup shaped base element (notshown) of carbon fibre material impregnated with sulphur. These elementslie between the electrolyte tube 14 and an outer metal case 1, thecathode elements 23 being in contact both with the beta aluminaelectrolyte tube 14 and the case 1. These cathode elements may be formedin the known way by compression of the fibre material which isimpregnated with hot sulphur, the sulphur then being cooled so as to besolidified and thereby to hold the element in compression to facilitateassembly of the cell. When the cell is raised to the operatingtemperature, typically 350°, the sulphur melts and the resilience of thefibre material causes the elements 23 to make good contact with the case1 and the electrolyte 14.

In this example of the invention, the case 1 is made from a steelsubstrate having a roll bonded coating of aluminium/silicon alloy on aninternal surface. The coating is plasma sprayed with nichrome.

The bulk analysis of the aluminium silicon alloy coating is, by weight:

0.9%--Fe

1.0-1.5%--Si

0.3%--Cu

0.2%--Mg

96.2%--Al

1.3%--O

The oxygen present is probably retained as an oxide of e.g. Al. Theremay typically be in addition up to 0.5% trace elements in theformulation. As a result of the roll bonding process, there is a layerof almost pure silicon on the surface of the alloy, resulting from thereleasing agent used during cold rolling. The casing 1 is highlyresistant to corrosion, and extremely strong. Importantly also, thecoating is resistant to the formation of a brittle intermetallic layerup to temperatures of about 615° C. Growth of intermetallics is bothtemperature and time dependent so that during relatively rapid hightemperature (approximately 600° C.) processing steps (e.g. bythermocompression bonding or resistance welding), the growth ofintermetallics is not a significant problem.

Samples of the roll bonded material have been maintained atapproximately 600° C. for 2 weeks and showed no substantial growth ofintermetallic alloys. At the operating temperature of an sodium/sulphurcell, i.e. approximately 350° C., samples have exhibited no sign ofsignificant intermetallics for substantially greater periods.Conveniently, at higher temperatures of approximately 700° C., thegrowth of intermetallics is sufficiently rapid so that significantquantities of aluminium are conserved from burning. This is particularlyadvantageous since an aluminium fire in a given cell is potentially aproblem for the entire group of cells.

The preferred choice of thickness for the aluminium/silicon alloycoating has been found to be approximately 60 micrometers thick. Athickness of 120 microns provides even better performance, extending thelife of the cell by as much as double.

The alpha alumina plate 15 is formed as a disc with a central aperture22. This disc is sealed to the case 1 by means of an annular metalmember 25 formed of the steel substrate with the roll bondedaluminium/silicon alloy coating, although Inconel 600 or Fecralloy A mayalso be used. This member 25 is secured by welding to the periphery ofthe housing and by thermocompression bonding to the disc 15 in anannular region around the centralaperture 22. The central compartment ofthe cell is closed by means of a current collector 21 passing throughthe aperture 22 and secured to an inner metal element 9 also bonded tothe alpha alumina around the aperture 22 by means of thermocompressionbonding. The element 9 is spaced radially inwardly from the annularmetal member 25 so that they are electrically insulated from one anotherby the alpha alumina disc. The element 9 may also be made of the steelsubstrate with the roll bonded aluminium/silicon alloy coating.

In the manufacture of the cell, the metal members 25 and 9 are bonded tothe alpha alumina end plate 15 before further assembly of the cell. Thisbonding is effected by compression at an elevated temperature and undervacuum conditions or in an inert atmosphere.

The inner metal member 9 is of relatively small radial extent and theseal is effected by applying pressure through a backing washer 10 toseal the outer peripheral edge of inner metal member 9. The material ofthe washer 10 is such as to become bonded to the member. The outerannular metal member 25 is sealed to the alpha alumina lid over a smallannular region around the inner member but slightly spaced therefrom byapplying pressure through a further backing washer 11.

A strengthening washer 8 is also thermocompression bonded to the uppersurface of the inner metal member, annular sheet 9. The strengtheningwasher 8 has a thickness greater than the thickness of the member 9 andserves to keep the inner peripheral portion of the sheet 9 substantiallyrigid. The outer diameter of the washer 8 is substantially less than thediameter of the backing washer 10, corresponding to the position of theseal between the member 9 and the alpha alumina lid 15.

The central current collector 21 extending through the aperture 22 hasan annular shoulder 7 which seals against the inner edge of thestrengthening washer 8 and is welded thereto to provide the necessaryhermetic seal.

Because the annular sheet 9 is bonded to the aplha alumina lid 15 onlyabout the outer periphery of the sheet 9, some flexibility is providedbetween the seal with the central current collector 21 and the seal tolid 15. The material of the sheet 9 is made sufficiently thin to permitsome distortion in the region indicated generally at 26 between thebacking washer 10 and the strengthening washer 8.

I claim:
 1. Alkali metal energy conversion device comprising sealingcomponents made of a metal or metal alloy substrate having a solid phasebonded coating of aluminium/silicon alloy on an internal surfacethereof, said aluminium/silicon alloy coating having the composition byweight of0-0.5% Mg, 0-0.4% Cu, 0-1.0% Fe, 0.5-4.0% Si up to 0.5% othertrace elements, some oxygen present as an oxide and remainder Al. 2.Alkali metal energy conversion device having an external casing, a solidelectrolyte element in the casing to divide the interior into electroderegions, an electrically insulating element joined to the electrolyteelement and means secured to the insulating element and the externalcasing to seal off one of the electrode regions, wherein at least partof the external casing is made of a metal or metal alloy substratehaving a solid phase bonded coating of aluminium/silicon alloy on aninternal surface thereof, said aluminium silicon alloy coating havingthe composition by weight of0-0.5% Mg, 0-0.4% Cu, 0-1.0% Fe, 0.5-4.0%Si, up to 0.5% other trace elements, some oxygen present as an oxide andremainder Al.
 3. Alkali metal energy conversion device as claimed inclaim 2 wherein the means secured to the insulating element comprisescomponents made of said substrate having said coating.
 4. Alkali metalenergy conversion device as claimed in any preceding claim wherein saidsubstrate having said coating has a thin layer of silicon on the surfaceof the aluminium/silicon alloy.
 5. Alkali metal energy conversion deviceas claimed in claim 1, 2, or 3 wherein said substrate is iron or nickelbased.
 6. Alkali metal energy conversion device as claimed in claim 5wherein said substrate contains chromium.
 7. Alkali metal energyconversion device as claimed in claim 5 wherein the substrate is steel.8. Alkali metal energy conversion device as claimed in claim 1, 2 or 3wherein said coating is a roll-bonded coating.
 9. Alkali metal energyconversion device as claimed in claim 1, 2 or 3 wherein said coating hasa thickness greater than 50 microns.
 10. Alkali metal energy conversiondevice as claimed in claim 9 wherein said coating has a thickness equalto or greater than 60 microns.
 11. Alkali metal energy conversion deviceas claimed in claim 10 wherein said coating has a thickness equal to orgreater than 100 microns.
 12. Alkali metal energy conversion device asclaimed in claim 8 wherein said coating has a thickness greater than 50microns.
 13. Alkali metal energy conversion device as claimed in claim12 wherein said coating has a thickness equal to or greater than 60microns.
 14. Alkali metal energy conversion device as claimed in claim13, wherein said coating has a thickness equal to or greater than 100microns.
 15. Alkali metal energy conversion device as claimed in claim 6wherein the substrate is steel.
 16. Alkali metal energy conversiondevice as claimed in claim 1 or 2 wherein the composition by weight ofMg is 0<% Mg≦0.5.